Navigate to Status → System Logs → System → DNS Resolver.

Many log message showing errors such as:

Mar 26 13:21:16 	unbound 	33572:0 	notice: sendto failed: Permission denied 

The resolver settings are not dynamically checking/applying upstream ipv4/6 connectivity but assuming both are active.

Enable or disable whether ip6 queries are answered or issued. Default is yes.

If disabled, queries are not answered on IPv6, and queries are not sent on IPv6 to the internet name-servers.

With this option you can disable the ipv6 transport for sending DNS traffic, it does not impact the contents of the DNS traffic, which may have ip4 and ip6 addresses in it.